A small change in word strength
If you've been using Duolingo for a few months, you might notice that some of your strength bars shifted up/down a bit today. That's because we made some adjustments to the word strength predictor.
This is a minor change, so most of you might not notice anything. But it fixes a bug that caused the words you know really well to stay at four bars (and unavailable for practice) for a long time... and then suddenly drop to one bar after a few months! Now everything degrades gradually like it should.
Words are bumped to 4 bars whenever you learn or practice them, but the decay rate depends on how often you get them right or wrong (so new words decay in a day or two, words you've gotten right 10+ times might take months). We're always tweaking things under the hood, and hope to have this perfected soon. :)
In other SRS programs I've used, such as Anki, the sorting algorithm is a bit more transparent. That is: the user can clearly see when individual facts will become due again. This might decrease complaints about word strength, or, at least, direct them in a more constructive direction.
There is definitely a huge improvement. I could "Practice Weakest Words" over and over again, and my tree would never return to gold. And lessons that did, would be back to blue or green again in a few days.
Today, I've been practicing weakest words off and on and if you take a look, my tree is almost completely golden again!